FutureTask实现异步的分析
1.FutureTask的类结构
首先咱们看下FutureTask的类型的继承关系,它同时实现了Future和Runnable的接口,也就是 具有了Future的异步的功能. 
promise
2. Future的定义
public interface Future<V> {
/**
* Attempts to cancel execution of this task. This attempt will
* fail if the task has already completed, has already been cancelled,
* or could not be cancelled for some other reason. If successful,
* and this task has not started when {@code cancel} is called,
* this task should never run. If the task has already started,
* then the {@code mayInterruptIfRunning} parameter determines
* whether the thread executing this task should be interrupted in
* an attempt to stop the task.
*
* <p>After this method returns, subsequent calls to {@link #isDone} will
* always return {@code true}. Subsequent calls to {@link #isCancelled}
* will always return {@code true} if this method returned {@code true}.
*
* @param mayInterruptIfRunning {@code true} if the thread executing this
* task should be interrupted; otherwise, in-progress tasks are allowed
* to complete
* @return {@code false} if the task could not be cancelled,
* typically because it has already completed normally;
* {@code true} otherwise
*/
boolean cancel(boolean mayInterruptIfRunning);
/**
* Returns {@code true} if this task was cancelled before it completed
* normally.
*
* @return {@code true} if this task was cancelled before it completed
*/
boolean isCancelled();
/**
* Returns {@code true} if this task completed.
*
* Completion may be due to normal termination, an exception, or
* cancellation -- in all of these cases, this method will return
* {@code true}.
*
* @return {@code true} if this task completed
*/
boolean isDone();
/**
* Waits if necessary for the computation to complete, and then
* retrieves its result.
*
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread was interrupted
* while waiting
*/
V get() throws InterruptedException, ExecutionException;
/**
* Waits if necessary for at most the given time for the computation
* to complete, and then retrieves its result, if available.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread was interrupted
* while waiting
* @throws TimeoutException if the wait timed out
*/
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
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能够看到Future是JDK1.5引入的异步的方式,而FutureTask则是异步的实现,它 提供了取消任务、检测是取消、是否完成、获取执行结构等操做, 其中获取结果若是 任务没有执行完成,会阻塞任务的执行.直到任务返回结果.markdown
3. 任务状态转换
全部可能的状态转换:
NEW(新建立) -> COMPLETING(完成中) -> NORMAL(正常结束)
NEW(新建立) -> COMPLETING(完成中) -> EXCEPTIONAL(异常结束)
NEW(新建立) -> CANCELLED(取消)
NEW(新建立) -> INTERRUPTING(中断中) -> INTERRUPTED(中断完成) \异步
4 FutureTask任务执行
public void run() {
if (state != NEW ||
!RUNNER.compareAndSet(this, null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
//任务正常结束,设置任务的返回值
set(result);
}
} finally {
// runner must be non-null until state is settled to
// prevent concurrent calls to run()
runner = null;
// state must be re-read after nulling runner to prevent
// leaked interrupts
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
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其实现异步的关键,在于最后任务执行完成, 将任务执行的执行的结果从新甚至回FutureTask的outcome字段,而后经过get方法能够获取异步任务返回的结果,this
5 FutureTask的get方法获取异步任务结构
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}
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能够看出,在调用get方法时, 若是任务state是小于COMPLETING,说明任务还在进行中, 则调用awaitDone进行阻塞等待任务执行,spa
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
// The code below is very delicate, to achieve these goals:
// - call nanoTime exactly once for each call to park
// - if nanos <= 0L, return promptly without allocation or nanoTime
// - if nanos == Long.MIN_VALUE, don't underflow
// - if nanos == Long.MAX_VALUE, and nanoTime is non-monotonic
// and we suffer a spurious wakeup, we will do no worse than
// to park-spin for a while
long startTime = 0L; // Special value 0L means not yet parked
WaitNode q = null;
boolean queued = false;
for (;;) {
int s = state;
if (s > COMPLETING) {
if (q != null)
q.thread = null;
return s;
}
else if (s == COMPLETING)
// We may have already promised (via isDone) that we are done
// so never return empty-handed or throw InterruptedException
Thread.yield();
else if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
else if (q == null) {
if (timed && nanos <= 0L)
return s;
q = new WaitNode();
}
else if (!queued)
queued = WAITERS.weakCompareAndSet(this, q.next = waiters, q);
else if (timed) {
final long parkNanos;
if (startTime == 0L) { // first time
startTime = System.nanoTime();
if (startTime == 0L)
startTime = 1L;
parkNanos = nanos;
} else {
long elapsed = System.nanoTime() - startTime;
if (elapsed >= nanos) {
removeWaiter(q);
return state;
}
parkNanos = nanos - elapsed;
}
// nanoTime may be slow; recheck before parking
if (state < COMPLETING)
LockSupport.parkNanos(this, parkNanos);
}
else
LockSupport.park(this);
}
}
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能够看出这里是经过是LockSupport.park进行现成阻塞,直到任务完成,线程
总结:
今天主要是分析JDK的Future实现类的FutureTask实现异步的方式,若是是向线程池提交任务,任务是有线程池的工做线程执行,而不会阻塞住线程, 可是JDK并非真正意义上的异步,由于提交任务后,当即执行get获取结果,依然是回同步阻塞的,而Netty的Promise 则是异步的回调的方式实现异步,是JDK的异步的加强,有兴趣的同窗能够看下它的实现,之后有机会会再次分享出来.code